Figure 1 | Figure 2 | Figure 3 | Figure 4 |
Table 1 | Table 2 | Table 3a | Table 3b |
GEOLOGICAL SURVEY
OF CANADA
OPEN FILE 4889
Apatite fission
track age, length and kinetic parameter (Cl, Dpar)
data for the
Northrock et al. East MacKay I-77 well,
central Mackenzie
Valley, Northwest Territories
D.R. Issler
Natural Resources
Canada, Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2L 2A7
and
A.M. Grist
Department of Earth
Sciences, Dalhousie University, Halifax, NS B3H 4J1
2005
Abstract
Apatite fission track (FT) age and
length measurements for a sandstone cuttings sample from the Devonian Imperial
Formation of the Northrock et al. East MacKay I-77 well (64o
46' 41.61" N latitude; 125o 43' 10.28" W longitude) are
presented herein. The well is located in the central Mackenzie Valley,
Northwest Territories, south of Tulita and east of the MacKay range. Also
included are compositional (electron microprobe elemental data) and etch figure
size (Dpar) data for apatite age and length grains that are used for defining
FT age populations with different thermal annealing behaviour. Two different
kinetic populations with pooled FT ages of 90.4±6.1 Ma and 222.2±22.5 Ma are
well defined on the basis of Cl content. A Cl value of 0.125 atoms per formula
unit (apfu) (0.45 wt%) separates these two FT kinetic populations whereas FT
ages show considerable overlap when plotted with respect to the Dpar parameter.
Introduction
This report presents supplementary
data and plots to accompany the paper by Issler et al. (in press)
on a multi-kinetic apatite fission track (FT) thermal history study of the East
MacKay I-77 well in the central Mackenzie Valley, Northwest Territories. Data
include apatite FT age (Table 1) and length (Table 2) measurements with
corresponding kinetic parameter data for a sandstone cuttings sample from the
depth interval, 1730-1780 mKB. Also included are elemental data for apatite
grains originally collected as wt % (Table 3a) and recalculated in terms of atoms
per formula unit (apfu; Table 3b). The I-77 well is situated in the Keele
Tectonic Zone (MacLean and Cook, 1999), a region with a history of anomalous
subsidence and exhumation throughout the Phanerozoic. The well penetrated Upper
Cretaceous foreland strata resting unconformably on Upper Devonian strata of
the Imperial Formation. The FT sample is interesting because it consists of two
age populations with different thermal annealing properties that permit
enhanced resolution of its post-Devonian thermal history. Very little has been
published on multi-kinetic apatite FT data; the I-77 data provide a
well-documented example.
Apatite FT Age Data
Table 1 lists apatite grain age
information along with the measured kinetic parameters, Cl content (from Table
3b), and Dpar, the arithmetic mean maximum diameter of FT etch figures parallel
to the crystallographic c-axis (Donelick, 1993; Burtner et al., 1994).
Data acquisition procedures are described in Issler et al. (in press).
Apatite composition is known to affect temperature-dependent FT annealing (e.g.
Green et al., 1986; Carlson et al., 1999; Barbarand et al.,
2003; Ravenhurst et al., 2003). Generally, increased Cl content causes
increased resistance to annealing except at very high concentrations where the
trend reverses (Carlson et al., 1999; Gleadow et al., 2002; Kohn et
al., 2002). Dpar depends on apatite solubility which in turn is a function
of apatite composition; therefore it is a proxy kinetic parameter for FT
annealing. Table 1 contains both measured and corrected Dpar values; the
correction is meant to compensate for differences in laboratory etching
conditions and observer bias so that the data are equivalent to those of
Carlson et al. (1999) for use with the Ketcham et al. (1999)
annealing model. Measured Dpar values are multiplied by the ratio of average
Dpar values for Durango apatite as reported by Carlson et al. (1999)
(1.83 mm) and as measured in this study (1.74 mm). The effect is
to increase measured Dpar values by approximately 5%.
OH content has been used as a
kinetic parameter for FT annealing by Ketcham et al. (1999) and
calculated OH values (from Table 3b) are included in
Table 1. The parameter, rmro,
is a measure of the relative resistance to annealing among apatite grains (Table
1). It forms the basis for the multi-kinetic FT annealing model which compares
the degree of apatite FT annealing relative to that for the most retentive
end-member apatite composition. It is defined as the reduced length of the most
resistant apatite at the point where the less resistant apatite first becomes
totally annealed. rmro values were calculated using the empirical
equation of Carlson et al. (1999),
![]() |
(1) |
which uses apfu
elemental values in Table 3b for a Ca10(PO4)6F2
end-member. Others is the sum of the cations Na, Mg, Sr, Y, Ce and La (Table
3b). rmro is also expressed in terms of Cl and Dpar using the
equations (Ketcham et al., 1999, 2000),
![]() |
(2) |
![]() |
(3) |
Calculated rmro
values from equation 1 were substituted into equations 2 and 3 to obtain
calculated Cl and Dpar values for comparison with measured values (Table 1).
A plot of apatite FT grain age
versus measured Dpar (Figure 1A) indicates that different age populations
cannot be resolved using Dpar; ages are grouped according to their similarity
and show considerable overlap with respect to Dpar. In contrast, a younger
(90.4±6.1 Ma) and older (222.2±22.5Ma) FT age population are resolved when
grain ages are plotted with respect to Cl content (Figure 1B). Three anomalous
high Cl grains share attributes of both populations and are likely caved from
the Cretaceous section. A plot of Cl versus Dpar for age (Figure 2A) and length (Figure 2B) grains
shows considerable scatter; the slopes of the linear regression equations
(calculated using the reduced major axis method; e.g. Davis, 1986) are very
similar to the relationship deduced from equations 2 and 3 except that the
lines are offset by approximately 0.2 mm on the Dpar axis. This may
indicate that our Dpar correction has not fully compensated for differences
between our measurement techniques and those of Carlson et al. (1999).
A plot of FT grain age versus number
of grid counting squares for track density measurement (Quads;
Fig. 3A and
Table 1) indicates that the older FT grains are associated to 16 or less grid
squares whereas the younger grains span from 4 to 80 grid squares. The number
of grid squares is a proxy measure for grain size; thus, older FT grains are
silt-sized whereas the younger grains vary from silt- to sand-size. Similarly,
high Cl content (with the exception of a few outliers;
Fig. 3B and Table 1) is
associated with silt-sized grains whereas lower Cl values occur over a broader
range of grain sizes. These results are in accord with the multi-stage sample
processing history; the older silt-sized fraction was recovered when the sample
was reprocessed with new plates on the rock crusher (Issler et al., in
press). The differences in age, Cl content and grain-size strongly
indicates that this sample contains different kinetic populations with
different thermal annealing behaviour.
Apatite FT Length
Data
Table 2 contains all the horizontal,
confined (etched FTs parallel and below the polished mineral surface) FT length
measurements, their orientation with respect to the crystallographic c-axis,
and the associated measured and calculated kinetic parameters (Dpar, Cl, OH, rmro)
as described above. Sixty lengths are associated with kinetic population 1
fluorapatite grains (mean length is 10.73±2.14 mm); kinetic
population 2 Cl-rich grains have 111 FT length measurements (mean length is
10.41±1.99 mm). Apatite grains for three additional lengths were
not probed but do have Dpar data (Table 2). Most of the length measurements
come from two separate grain mounts, irradiated with 252Cf, to
increase the number of etchant pathways for enhanced revelation of horizontal
tracks. Additional FT lengths were obtained from three different age mounts
(Issler et al., in press).
FT annealing is anisotropic (e.g. Green
et al., 1986; Galbraith and Laslett, 1988; Galbraith et al., 1990;
Donelick, 1991) and therefore horizontal FT lengths can be corrected back to a
standard orientation (parallel to crystallographic c-axis) using an appropriate
length projection model (Donelick et al., 1999; Ketcham, 2003). A plot
of measured FT length (Fig. 4A) and FT lengths projected parallel to the
mineral c-axis (Fig. 4B) versus Cl shows little structure in the data,
consistent with two kinetic populations that experienced similar degrees of
annealing but at different temperatures and times (see modelling results of
Issler et al., in press). For modelling purposes, the measured
track lengths in Table 2 were corrected by Issler et al. (in press)
in a similar manner to the Dpar measurements as described above.
Apatite Elemental Data
Table 3a lists the raw elemental wt%
data obtained from the JEOL electron microprobe at Dalhousie University. Single
spot analyses were done using wavelength-dispersive methods with an accelerating
voltage of 15 kV, a beam current of approximately 20.6 nA and a beam width of 5
mm
(an initial beam width of 10 mm yielded low elemental totals
for the silt-sized apatite grains). With a few exceptions, duplicate analyses
for selected grains showed good reproducibility (generally 0.01-0.05 wt% for Cl
values <0.6 wt%). Table 3a includes some low elemental totals but these data
were still useful for sorting the apatite grains into high Cl and low Cl
populations. Despite the wide variation in Cl content for kinetic population 2
(0.45 to 2.25 wt%), an effective Cl concentration of 0.21 apfu (0.71 wt%) could
be used to model the data (Issler et al., in press). This range
in Cl content is far greater than the uncertainty in analytical precision on
single grain measurements.
Elemental wt% data were converted to
apfu (Table 3b) in a spreadsheet assuming a 42 atom total based on the
end-member formula, Ca10(PO4)6F2,
full occupancy of the halogen site (OH determined by iteration using measured
O) and various cation substitutions. We used the following standard procedures:
(1) elemental wt% values were converted to atomic proportions by dividing by
the atomic weight; (2) apfu values were calculated by dividing the atomic
proportions by the sum of atomic proportions and multiplying by 42 atoms; (3)
OH was calculated iteratively using the criterion, F+Cl+OH=2; (4) O values were
adjusting according to the amount of O incorporated into OH. The quality of
analyses was judged using the wt% totals and the total for cations and anions
based on the idealized apatite formula. Also listed in
Table 3b are the kinetic
parameters, rmro, Cl and Dpar, calculated using equations 1 to 3.
Acknowledgements
We
thank Art Stirrett of Northrock Resources Ltd. for encouraging us to undertake
this study and for supplying geological information. We also thank Ray Donelick
for granting us permission to use his patented Dpar technique. Kim Dunn is
thanked for her technical assistance in the final production of this report. The
study was funded by Northrock Resources Ltd. and the Program on Energy Research
and Development (PERD) under the Northern Resources Development Program of the
Geological Survey of Canada..
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